I think the 18" oven is kind of small. The 36" oven - and all these designating dimensions are inside diameters - has been our best seller. But now we have new 24" and 48" ovens and they are really sweet.

We built a 36" oven in our office thinking that the 24" was just too small to cook more than one pizza at a time with a fire in the oven. Turns out that we can't make the pizzas fast enough to get more than one in at a time. At 800 to 1,000 degrees F, our pizzas are done in 90 seconds - faster than we can make them.

So maybe it depends on how much room you have. Any of the sizes, with 8" thick walls, need a platform at least 16" larger than the diameter. A 24" oven needs at least 40", a 36" oven needs 52", etc.